Going to have to second this, along with Dragonball Z, which might be one of the most parodied animes I've seen. Any of the major "power-up" scenes would work. To contribute a bit to the thread, and add one that some might forget about (while still being iconic, if that's possible), I'll add a bit of Kenshin:

Spoiler Alert! Click to show or hide

Edit: That Ghost in the Shell pic is another one I'd place way up on the list.

Edit 2: It's a bit too recent to be considered "iconic," but Madoka Magica is well on its way to that sort of status as well: